Powershift transmissions are used in a variety of work machines where it is desirable to shift gears without interrupting the torque flow through the transmission. To this end, powershift transmissions generally have a clutch overlap period whereby both an off-going and an on-coming clutch are engaged at the same time. If this overlap period is too short, then there will be a torque interruption to an output shaft prior to the engagement of the on-coming clutch. If the overlap period is too long, then the shift will appear to be slow and the transmission will be exposed stresses which can potentially reduce the life of various transmission components.
In addition, a single overlap period may not be desirable for all load conditions. In particular, under high load conditions, it is desirable to increase the overlap period to effectively complete the gear shift. On the other hand, under light load conditions, it is desirable to decrease the overlap period to effectively complete the shift. Moreover, the rate in which the on-coming clutch is engaged may also be varied to improve the shift. Under high load conditions, it is desirable to increase the rate at which the on-coming clutch is engaged whereas under low load conditions, it is desirable to decrease the rate which the on-coming clutch is engaged.
Another problem may arise during double clutch shifts where a pair of on-coming clutches are engaged to accomplish a gear shift. During double clutch shifts, the inertia of a first clutch may be greater than the inertia of a second clutch. It may be desirable to engage the first clutch earlier than the second clutch to compensate for the change in relative inertia of the clutches. Similarly, under different load conditions, it may be desirable to engage the first clutch later than the second clutch to compensate for the change in relative inertia of the clutches.
What is needed therefore is a method and apparatus for adaptively engaging clutches based on engine load which overcomes the above-mentioned drawbacks.